Real Number -- from Wolfram MathWorld

The field of all rational and irrational numbers is called the real numbers, or simply the "reals," and denoted R. The set of real numbers is also called the continuum, denoted c. The set of reals is called Reals in the Wolfram Language, and a number x can be tested to see if it is a member of the reals using the command Element[x, Reals], and expressions that are real numbers have the Head of Real. The real numbers can be extended with the addition of the imaginary number i, equal...